// let intervalID = setInterval(myCallback, 1000);

// function myCallback() {
//     console.log(Date.now());
// }

let intervalID;
function startChange() {
    if (!intervalID) {
    intervalID = setInterval(changeColor, 1000);
    }
}

function changeColor() {
    if (document.body.style.backgroundColor !== "black") {
        document.body.style.backgroundColor = "black";
        document.body.style.color = "white";
    } else {
        document.body.style.backgroundColor = "white";
        document.body.style.color = "black";
    }
}

function stopChange() {
    clearInterval(intervalID);
   }

document.getElementById("start").addEventListener("click", startChange);   
document.getElementById("stop").addEventListener("click", stopChange);
